1. Field of the Invention
The present invention relates to the control of the flow of fluid in valves and, more particularly, to a valve arrangement offering stability over a range of fluid flow rates.
2. Description of the Prior Art
Governor valves are known for use in controlling the operating pressure in fluid systems, such as steam turbine systems. In a steam turbine system, the inlet port of the steam turbine is connected to the outlet port of a governor valve that regulates or controls the fluid flow entering the steam turbine.
It is known in the art that steam turbine governor valves often encounter flow instabilities at some operating positions (i.e., lifts) which can cause noise, excessive vibration, and, possibly, valve failure. Steam turbine governor valves typically employ a cutoff plug that engages a valve seat to regulate the steam flow. The valve seat typically resembles a portion of a toroid. The cutoff plug in operation moves along the valve central axis of the toroid to block or allow fluid movement through the toroidal opening. The seat throat is the most constricted part of this opening.
Flow induced instability is believed to be the most likely cause of catastrophic failures in steam turbine governor valves known in the prior art. Steady state forces are inadequate to explain the damage observed in practice in the prior art. Fracture analysis indicates the occurrence of cyclic loading, which indicates the presence of a time-varying force; and frequency analysis shows no likely simple resonances.
Flow patterns through valves vary with valve design and flow volume, FIGS. 1A and 1B illustrate the differences between annular flow and core flow patterns. In the desirable annular flow pattern there is no impingement of the jet flow in the center of the valve outlet. Instead, an annulus or flow ring is produced wherein upward flow is produced in the center of the valve outlet beneath the plug, and downward flow is restricted to the interior walls of the valve outlet thereby “adhering” or “attaching” to the walls. In the undesirable core flow pattern, the resulting sheet of jet flow impinging on itself is unstable, producing a shifting pattern beneath the plug. The rotating bending predicted to result from shifting jet impingement is consistent with valve stem fracture analysis.
FIGS. 2A and 2B and 3A and 3B contain plots from computational fluid dynamics (CFD) analyses of two prior art plug designs. Flow is downward in the plots. Streaklines show the pattern of flow downstream from each plug. The region in the center of each plot is the reverse flow region that occurs with annular flow.
Referring to FIGS. 2A and 2B, cutoff plugs in which the plug surface cuts away abruptly (i.e., has a sharp cutoff corner) are known to work well at low lift positions. The cutoff corner is located at the tip of the cutoff plug and may generally be defined as the angle formed between the sidewall and the bottom of the cutoff plug. The flow separates from the plug at the cutoff corner and follows the seat wall in an annular form. However, at high lifts (i.e., high flow rate) the opening between the cutoff plug and the seat wall is large compared to the seat radius of curvature. Under these conditions, the seat is unable to guide the flow downward in an annular form with the flow remaining attached to the seat wall. A sheet of unstable flow from the seat wall to the center of the toroid opening beneath the cutoff plug results. This flow pattern is unstable and produces turbulence in the fluid flow volume beneath the cutoff plug.
U.S. Pat. No. 5,312,085 to Yokoyama et al. depicts a typical control valve incorporating a plug with a sharp cutoff corner. As the Yokoyama et al. valve is opened, the location of the plug-seat throat changes. The plug-seat throat is the location at which the cross section of the flow path is at a minimum. The plug-seat throat is therefore the narrowest opening between the plug and the seat. The shutoff valve on which the plug is mounted abuts a valve seat at a contact point. As the plug lifts, the plug-seat throat shifts from the contact point high on the plug and seat to a sharp corner located near the narrowest opening of the seat, but of a smaller diameter than the seat so the sharp corner can never contact the seat. The plug-seat throat remains at the sharp corner as the valve opens more fully. This controlling sharp corner is located on the end of a flange that is of larger diameter than the shaft it is mounted in. This causes the flow passage at low lifts to be narrow at the sharp corner, widen quickly, and then become narrow again near the contact point. This feature causes the high pressure drop experienced at low lifts to be taken in two stages and prevents cavitation because this valve is intended for liquid service rather than for steam.
A flowpath that has two throats (i.e., two narrow constrictions in series) is a critical feature of the Yokoyama et al. valve, but is inadvisable in a valve designed to control the flow of steam. Steam is a compressible fluid and a flowpath incorporating two throats in series is a well known source of flow instability in compressible flow. Additionally, the Yokoyama et al. valve does not resolve instability problems in steam turbine governor valves resulting from the flow impinging on itself beneath the plug. The direction of flow in steam turbine governor valves is from the plug-seat contact point to the narrowest opening of the seat. The direction of flow in the Yokoyama et al. valve is from the narrowest opening of the seat to the plug-seat contact point. The volume in the Yokoyama et al. valve analogous to the volume under the plug in a steam turbine governor valve is occupied by a solid shutoff valve so there is no flow in this volume.
Referring to FIGS. 3A and 3B, cutoff plugs can also be shaped to guide the flow downward in an annular form. For example, cutoff plugs with concave flow surfaces are known in the art. This configuration works well at lifts where the bottom of the plug is level with the seat throat or at higher lifts. At these high lifts, the flow separates from the plug at the cutoff corner and follows or “attaches” the seat wall. At lower lifts (i.e., low flow rate), however, the flow follows the plug wall until the cutoff corner, being guided away from the seat wall. The flow still leaves the plug in an annular pattern but shortly thereafter separates from the seat wall, which dissipates the annulus quickly. FIGS. 3A and 3B show, respectively, the small central reverse flow region at low lift and large reverse flow region at high lift that is characteristic of a “concave” cutoff plug. Annular flow patterns tend to be stable producing a uniform force on the plug. Flow patterns in which the flow impinges on itself beneath the plug are chaotic producing forces on the plug that vary rapidly. It is therefore desirable to use a plug and seat configuration in which the flow takes an annular form at all lifts.
U.S. Pat. Nos. 4,688,755 and 4,735,224 to Pluviose depict a control valve incorporating a plug with a somewhat concave flow surface. Though a portion of the lateral surface of the plug is concave, the lateral surface terminates in an extended cylindrical portion forming a right angle with the end or tip of the plug. Because of this terminal cylindrical portion, the throat of the opening between the seat and the plug is not in proximity to the cutoff corner. The cutoff corner is therefore unable to cause the flow to follow or “attach” to the seat wall, and the desirable annular flow pattern is not produced. The control valve disclosed by the Pluviose patents is configured to form two contiguous flow streams having different velocity distributions. This is accomplished by providing a valve seat with six uniformly spaced longitudinal recesses. The longitudinal recesses produce subsonic jets interposed between supersonic jets giving rise to an intense mixing process, stabilization of the jets, reduction of the lengths of the jets and noise attenuation. FIGS. 3A and 3B show, respectively, the small reverse flow region at low lift and large reverse flow region at high lift that is characteristic of a concave plug.
In summary, control valves having sharp cutoff corner plugs are known to work well at low flow—low lift conditions, but do not operate effectively at high flow—high lift conditions. Conversely, control valves having concave cutoff plugs are known to work well at high flow—high lift conditions, but do not operate effectively at low flow—low lift conditions. Accordingly, an object of the present invention is to provide a valve arrangement that operates effectively at both high and low lift operating conditions. It is also an object of the present invention to provide a valve arrangement that reduces instability of fluid flow in control valves that operate at high flow rates and operating pressures. Furthermore, it is a specific object of the present invention to combine advantageous features of cutoff plugs having abruptly cut-away plug surfaces and the advantageous features of cutoff plugs with concave flow surfaces in a single cutoff plug and valve seat arrangement.